Standard deviation
A measure of how spread out a set of numbers is around their mean.
Standard deviation measures the typical distance of values in a dataset from their mean. It is the square root of the variance. A small standard deviation means the data clusters tightly around the average; a large one means it's spread out. Use the population formula (divide by N) when you have the whole group, and the sample formula (divide by N−1) when your data is a sample of a larger population.